The other thing I was thinking is this... the collars are half way up the tube. The net pots bases are I guess about half way down the tube. This is critical initially to stop any dry zones between the plant and the root entrails. I guess he gets air in the zone within the net pot and when root growth builds many will lay on the top of the flow of water.
What I'm saying is I am sure that oxygen uptake could be greatly improved and if this is the case bear in mind the prolific yields he's getting from minimal watts, the sky is the limit!